How to Fill Concrete Gaps

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Fill large gaps or cracks in concrete to avoid damage to your slab.
Fill large gaps or cracks in concrete to avoid damage to your slab. (Image: Thinkstock/Comstock/Getty Images)

Gaps in concrete, whether they result from cracks in the slab or occur between the slab and an adjacent surface, can serve as openings that allow dirt, moisture or insects into your home. So filling them should be a maintenance priority. There’s no need to pay a contractor, however. You can fill most gaps yourself by sealing them with a urethane caulk that keeps anything from entering while maintaining flexibility that prevents cracking with concrete movement.

Things You'll Need

  • Wire brush
  • Stiff-bristle broom
  • Urethane caulk
  • Caulking gun
  • Utility knife
  • Foam backer rod
  • Spoon

Small Gaps

Clean the gap of any foreign material using a wire brush. Start at one corner of the gap and push the brush through, using a scooping motion with the brush along the gap’s length to pull any debris from the location. Remove the debris with a stiff-bristle broom.

Examine the size of the gap to determine the repair method. For gaps 1/4-inch or less in width, caulking alone will be sufficient to prevent anything from getting through. Use a backing rod to help in filling gaps of a larger size.

Place the tube of urethane caulk into a caulking gun. Press the trigger a few times to move the rod of the gun against the rear of the tube, locking it in place. Cut the tip of the tube with a utility knife to open it. Use an angled cut, about 45 degrees, and then turn the tube so that the angle points downward when applying the caulk.

Position the caulking gun at the start of the gap angled 45 degrees for smooth application of the caulk. Pull the trigger to force the caulk from the tube and into the gap. Follow the gap with the tube, slightly over-filling the gap with the caulk until you reach the gap’s end.

Flatten the caulk over the gap using the rear of a spoon. Wet the spoon in water to help it glide smoothly across the caulk surface.

Large Gaps

Measure the width and length of gaps wider than 1/4-inch. Cut a piece of foam backer rod that’s slightly larger in diameter than your gap width to the same length as the gap.

Stuff the rod into the gap, filling it up to about 1/4-inch from the surface of the concrete. If necessary, use multiple rods. Use the end of a putty knife to push the rod firmly into the gap.

Fill the space surrounding the rod with the caulk, over-filling the gap as you would with smaller cracks. Smooth the caulk out over the concrete surface with the rear of a wet spoon. Allow the caulk to dry for 48 hours after application.

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